The present invention relates to the field of synthetic aperture radar (SAR), and in particular to a SAR system having a rotating antenna.
German Patent Document DE-PS 39 22 086 discloses a rotating antenna synthetic aperture radar (ROSAR Device), in which at least one antenna for transmitting and receiving radar pulses is arranged on the end of a rotating arm (for example, a helicopter rotor or a turnstile above the rotor axis). The received signals are demodulated and stored intermediately, and are then correlated with reference functions which are each calculated and preset based on the illumination geometry of the radar device.
The parameters for calculating and presetting the reference functions are the distance intervals to be measured, the transmission frequency, the length of the rotating arm: the angle of rotation range of the antenna from which the reflected signals are received; the number of transmitted pulses as well as the height of the rotating antenna above the ground. The correlation result is appropriately displayed, for example, on a monitor.
A radar device of this type may be used in approximate real time, in online operation. It thus can be used, for example, not only in the field of cartography and in obstacle warning operations, but also for the purpose of target reconnaissance and tracking. The processor of this known ROSAR-device has several modules so that the multiple and complex computing tasks may be subdivided, thereby permitting approximate real time and on-line operation.
In this known apparatus, the result for each distance interval is always obtained by correlation of the received signal with a single reference function that is valid for that distance interval.
The resolution of a ROSAR-device in the lateral and the radial direction is determined by parameters which are partially interdependent; that is, by the wavelength .lambda. and the length L of the rotating antenna arm, by the apex angle of the antenna .gamma.; by the distance R.sub.GO between the antenna and the center line of the illuminated strip; by the height H.sub.o of the antenna above the ground: by the pulse repeating frequency .function..sub.p and the duration .tau. of the transmitted pulses, and therefore the number Z.sub.s of the pulses for each aperture length S: by the duration of the received echo signal: by the scanning rate for the distance intervals, etc.
Strictly speaking, in order to achieve a proper image in a system such as described above, a separate reference function would have to be calculated for each object point in the radial direction, and the received signal would have to be correlated with it. The fact that existing computing capacity is not unlimited is of course sufficient reason why such a computation is impossible. Thus, in the above-mentioned Germany Patent Document DE-PS 39 22 086, the calculated reference functions are each used for one distance interval, and the reference functions are then actually only valid for object points on the center of the distance interval. The correlation of the received signal from this distance interval can therefore be achieved only with the acceptance of a phase error. Although the resulting lack of definition may be reduced by selection of correspondingly small distance intervals, such a measure would then again require increased computing capacity.
It is an object of the present invention to provide a ROSAR-device of the initially mentioned type in which image defects are minimized by simple means.
This and other objects and advantages are achieved according to the invention, in which the processor circuit is expanded by an additional module, in which the valid definition of the image is calculated. The calculated definition of the image may then be used to simplify tile computing operation. The definition of the image indicates the frequency of a new calculation of the reference functions concerning the strip width of the individual distance intervals to be imaged.
As used herein, "definition of the image" refers to that area at whose edges two objects are still imaged with sufficient clarity. This valid range may be indicated mathematically on the boundaries by the distances R.sub.GO1 and R.sub.GO2 at .DELTA.R.sub.GOV =R.sub.G02 -R.sub.G01, in which case, for this range, a maximal phase difference of .pi./4 is permissible, corresponding to a path difference of .lambda./8. This relationship may be calculated as a function of the apex angle of the antenna .gamma. and its height H.sub.0 above the ground. Since in this case the calculation does not take place with the corresponding distance cells, the run index n is omitted: instead, the calculation takes place by means of R.sub.G01 and R.sub.G02.
The ROSAR-device according to the invention includes an additional memory module in which a calculated definition of the image is stored for different values of the parameters .gamma. and H.sub.o as a function of the respective distance R.sub.G01. Thus, on the basis of the actual circumstances (particularly the height of the antenna above the ground and the depression angle, that is, the angular field of the antenna in the direction of the ground), these stored values may then be used to read the appropriate range of the definition of the image, and to feed it to the processor circuit to subdivide the illuminated area into individual distance intervals, and thus subdivide the reference functions. The storage of the given ranges of definition of the image in this manner eliminates the need to calculate them in real time, which is particularly important because such calculation requires high expenditures.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.